The present invention relates to the monitoring of chemical processes and materials, and more specifically to monitoring by the determination of optical scattering properties.
A particular application of the present system is to monitor the condition of polymers during their processing. Such polymer processing yields polymer solutions which change as the processing proceeds. It is therefore necessary to monitor the polymer solution as processing proceeds, so that the progress of the processing can be followed and the processing stopped or modified at the appropriate time, or subsequent processing steps may be modified to account for the condition of the polymer solution.
An established monitoring technique for this is to use light scattering. In one known system, for example, a sample of the solution is placed in a cell through which a laser beam is passed, and three photodetectors are arranged to detect light scattered at three respective angles (approximately 45xc2x0, 90xc2x0, and 135xc2x0 to the laser beam).
In the context of polymer processing, it is conventional to take samples from the reaction vessel containing the solution at suitable intervals. Each sample is placed in a cell and its optical properties measured. The plant operator can therefore follow the progress of the mix and control it accordingly.
We have realized that this technique has a number of disadvantages, both apparent and real.
One disadvantage is that the temperature of the sample is likely to change, from the temperature of the reaction vessel to room temperature, before its properties are measured. However, the properties of the sample will generally change consistently with temperature, and the temperature of the reaction vessel will normally be measured anyway, so it will sometimes be possible to make allowances for this problem.
Another disadvantage with off-line sampling is that the solution tends to lose solvent during removal and testing, which changes the solvent: polymer ratio and the properties of the product being tested.
Further problems are that an operator must be available to take the sample and that there will be a time delay between taking the sample and determining its properties. These problems will make the operator""s task more difficult, since the taking of the samples will distract the operator and the time delay will result in the operator being given somewhat out-of-date information.
According to this aspect of the invention, there is provided a monitoring system for a reaction vessel for polymer processing and the like, comprising a monitoring cell for measuring the optical properties of the contents of the reaction vessel, a feed path and a return path both connecting the cell to the reaction vessel, and a pump connected in the feed or return path.
A significant advantage of this arrangement is that laser speckle in the monitoring cell is effectively eliminated, because the liquid being monitored is moving through the cell. No signal smoothing arrangements are therefore required.
This arrangement also makes the monitoring data available immediately, and ensures that the liquid being monitored is at substantially the same conditions (temperature and pressure and the solvent:polymer ratio) as in the reaction vessel. The fact that the temperature is maintained also means that liquids which would set or gel or otherwise be inconvenient to monitor at room temperature can be monitored without difficulty.
Turning to another aspect of optical liquid monitoring, the conventional arrangement is, as mentioned above, to pass a laser beam through the monitoring cell and provide one or more photodetectors arranged at suitable angles to the laser beam. One known system uses a single photodetector which detects backscattered light at an angle of 153xc2x0; dual angle systems are known; and another, as mentioned above, uses three photodetectors at angles of approximately 45xc2x0, 90xc2x0, and 135xc2x0.
The invention may be used to monitor polymers which are in solution, at least partially, or which are carried in liquid as a dispersion or emulsion, but not for melted polymers.
A wide variety of materials may be monitored, and the number and arrangement of photodetectors will normally be chosen in dependence on how far the particular materials to be monitored are likely to vary. If the materials to be monitored are tightly determined and their optical properties are well known, a single photodetector may be satisfactory; if the materials vary more widely, then two or three photodetectors may be used.
For a sample of relatively uniform material, the scattering light intensity curve (obtained by plotting the scattering light intensity against the angle of detection) is likely to have a fairly smooth curve shape. The position and height of the peak of the curve can of course both vary substantially, and the steepness of each of the sides of the curve can also vary. The selection of the positions of the photodetectors can therefore present difficulties, even with the use of three photodetectors. Further, there may be situations where there may be mixtures of different materials, in which case the curve may have a flattened peak or there may even be two or more curves. This can be overcome by providing more photodetectors, but that increases the cost and complexity of the monitoring cell.
The general object of this aspect of the invention is to alleviate the difficulties in selecting the number and positions of the photodetectors.
According to this aspect of the invention there is provided a monitoring cell for measuring the optical properties of the contents of the cell, including a source for generating a laser beam through the cell, and a detector for detecting light scattered from the laser beam, preferably a linear camera and an optical system for focusing scattered light thereon.
With this arrangement, the light scattered at any particular angle or set of angles can immediately be determined, by suitable sampling of the output scan of the camera. If desired, however, a complete graph of the scattered light against scattering angle can automatically be generated. If, for example, a modified or new polymer process is being developed, such displays can be generated for the entire range of polymer conditions occurring during the development of the process, and can then be analysed and compared and a suitable set of monitoring angles chosen which will give good discrimination between polymer conditions which need to be distinguished.
A polymer processing system including a monitoring system embodying these aspects of the invention will now be described, by way of example, with reference to the drawings, in which: